Cadillac Repair: no heat, coolant level, heater cores

Question
I get little if any heat from the heater. I checked the radiator. All OK, but after driving 40 min coolant warm, but not hot. Could it be that the thermostat is stuck open?

My gas mileage has also dropped. Warmer days lets the heater put out warmer air.

Answer
Hi Dave, That would be the first thing I would do after checking the coolant level is replace the thermostat as it could be stuck open. Heater cores get plugged and programmers fail to go to the full heat position also. Put in the new stat and your problems might be over. Bill